The present application relates to measuring radiation attenuation by an object exposed to radiation. It finds particular application in the field of computed tomography (CT) imaging utilized in medical, security, and/or industrial applications, for example. However, it also relates to other radiation imaging modalities where converting radiation energy into electrical signals may be useful, such as for imaging and/or object detection.
Today, CT and other imaging modalities (e.g., mammography, digital radiography, etc.) are useful to provide information, or images, of interior features of an object under examination. Generally, the object is exposed to polychromatic radiation comprising photons (e.g., such as x-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by the interior features of the object, or rather a number of radiation photons that are able to pass through the object. Generally, highly dense features of the object absorb and/or attenuate more radiation than less dense features, and thus a feature having a higher density, such as a bone or metal, for example, will be apparent when surrounded by less dense features, such as muscle or clothing.
The detector array typically comprises a plurality of detector cells, respectively configured to convert detected radiation into electrical signals. Based upon the number of radiation photons detected by respective detector cells and/or the electrical charge generated by respective detector cells between samplings, images can be reconstructed that are indicative of the density, effective atomic number (z), shape, and/or other properties of the object and/or features thereof.
Conventionally, radiation imaging systems employed a single energy scanner, which measures the attenuation of an integrated radiation spectrum and provides the density information of the object or rather features comprised therein. Using this density information respective features can be identified and/or classified (e.g., as a potential threat or non-threat item). While measuring the densities of the features has proven to be a useful tool for identification of the features, density information is sometimes insufficient. For example, some items of interest (e.g., threat items, tumors, etc.) may have substantially similar densities and shapes as items that are not of interest, which may make it difficult to identify some items based merely upon the measured density.
More recently, some radiation imaging systems have begun to use dual-energy scanners, which measure both the density and effective atomic number (z), of features within the object. In this way, items can be identified and/or classified based upon density and/or chemical makeup information, for example. Applications for dual-energy scanners may comprise, but are not limited to, bone densitometry, explosive detection, and/or quantitative computed tomography (CT).
Dual-energy imaging systems generally measure the absorption characteristics of features within the object under examination for a plurality of energy spectra (e.g., a higher energy spectrum and a lower energy spectrum). This approach is made possible because radiation undergoes different types of interactions with matter at different energies. In the diagnostic range of radiation energies up to 200 keV, for example, radiation interacts with matter primarily through Compton scattering and photoelectric interactions. These two types of interactions depend differently on the energy of the incident radiation. The cross-section for Compton scattering is proportional to the electron density of the object, while the photoelectric cross-section is proportional to the electron density times the atomic number cubed. Thus, by separately measuring radiation attenuation at two or more different energy spectra, the Compton scattering and photoelectric interactions can be independently measured. Based upon these independent measurements, density and effective atomic number (z) for items comprised in the object under examination can be determined.
One technique for obtaining such measurements is known as “source switching.” In source switching, the energy spectrum of the radiation is switched between at least two distinguished or different energy spectra. This may be done through a variety of procedures. In one procedure, the voltage applied to a radiation source is varied causing the emitted radiation's energy to vary with the change in voltage. In another procedure, two or more spatially separated sources are configured to alternate radiation emissions (e.g., by alternating power to the sources). Where there are two energy sources, for example, one of the sources may be configured to emit radiation within a first, higher energy spectrum while the other may be configured to emit radiation within a second, lower energy spectrum.
Another technique uses a dual-energy, indirect conversion detector array (e.g., generally of sandwich type design) that comprises two scintillators and two photodetectors. A first scintillator and photodetector are configured to measure object attenuation at a first effective photon energy (e.g., where the first effective photon energy corresponds to a mean energy detected by the first scintillator) and a second scintillator and photodetector are configured to measure object attenuation at a second effective photon energy (e.g., where the second effective photon energy corresponds to a mean energy detected by the second scintillator).